Recently, in order to increase the density of optical recording mediums (e.g. optical disks), microfabrication technologies for guiding grooves and prepits (often called “pits”) of optical disks have been developed. In general, the guiding grooves and prepits are formed by so-called mastering process. In the mastering process, photoresist is applied onto a glass substrate, and a laser beam is radiated and focused onto the photoresist. By thus exposing and developing the photoresist, an optical disk master copy is fabricated.
The diameter of the light beam spot (hereinafter “light spot”) of the focused laser beam is approximately 0.8λ/NA, where λ is the wavelength of the laser beam, and NA is the numerical aperture of the object lens that focuses the laser beam. In order to attain microfabrication of the guiding grooves and prepits of optical disks, it is necessary to reduce the diameter of the light spot. For this purpose, it is necessary to shorten the wavelength λ and to increase NA.
However, NA is already 0.9 or higher, which is close to the current upper limit. Therefore, it is virtually difficult to reduce the diameter of the light spot by further increasing NA. On the other hand, if such λ that is in the ultraviolet region is adopted (which means shortening the current wavelength), costs for optical members increase. In addition, this decreases the focal depth, thereby imposing stricter requirements to a master copy exposing device.
Among the methods of forming such guiding grooves and prepits that are smaller than the diameter of the light spot, there is an etching method. In the etching method, a photoresist pattern formed by development is used as a mask, and the material under the mask is etched. According to this method, such a part of the material under the mask that corresponds to the bottom width of the photoresist pattern is etched. Therefore, it is possible to form such guiding grooves and prepits that are equal to or smaller than the pattern formed in the photoresist.
Publication 1 discloses the following manufacturing method in which the photoresist pattern is used as a mask. First, a lower layer made of non-photosensitive water-soluble resin is formed on a glass substrate. Then, an intermediate layer made of inorganic material is formed on the lower layer. On the intermediate layer, an upper layer made of photosensitive resin is formed. Next, the upper layer is exposed and developed, so that a predetermined pattern is formed in the upper layer. At this time, reflecting the Gaussian distribution of the intensity of the light beam, the pattern in the upper layer is wider at an aperture than at a bottom.
Next, by etching the intermediate layer through the pattern in the upper layer, an aperture is formed. Then, optical ozone ashing is performed by using the intermediate layer as a mask. As a result, the pattern is transferred to the lower layer. Here, the width of the pattern in the upper layer determines the shape of the pattern in the lower layer. Therefore, it is possible to form such patterns of prepits and guiding grooves that are equal to or smaller than the diameter of the light spot. In addition, because the intermediate layer is made of inorganic material, the intermediate layer remains intact even after the upper layer (which is made of photosensitive resin) is etched by the optical ozone ashing. Therefore, such problems as the decay of an edge of a land part do not occur.
Another method of forming such guiding grooves and prepits that are smaller than the current diameter of the optical beam is disclosed in publication 2. The method of publication 2 is a method of recording information in an optical disk master copy. In this method, an electron beam is radiated onto the optical disk master copy, thereby forming a depressed part that represents recorded information.                (Publication 1) Japanese Publication for Unexamined Patent Application, Tokukai 2002-334483 (publication date: Nov. 22, 2002)        (Publication 2) Japanese Publication for Unexamined Patent Application, Tokukaihei 09-44847 (publication date: Feb. 14, 1997)        
However, the methods of publications 1 and 2 for forming micropatterns such as guiding grooves and prepits have the following problems: (1) there is a limit in forming such guiding grooves and prepits whose track pitch and pit pitch are smaller than the diameter of the light spot, or (2) a very expensive manufacturing device is required.
The problem of the method of publication 1 is as follows. In order to increase the density, it is necessary to fabricate narrow guiding grooves and small pits, and to shorten (i) the track pitch, i.e. groove-to-groove or pit-to-pit distance in the radial direction, and (ii) the pit pitch, i.e. pit-to-pit distance in the in-track direction. If the track pitch or pit pitch is smaller than the diameter of the light spot, light spots overlap on the photoresist. The method of publication 1 requires a contrast between an exposed part and a non-exposed part, so as to use the upper layer (which is made of photosensitive resin) as a mask layer in etching the intermediate layer. However, if the light spots overlap, the necessary contrast cannot be attained. Therefore, as microfabrication advances, it becomes impossible to fabricate such track pitch and pit pitch that are smaller than the diameter of the light spot.
For example, according to publication 1, the patterns can be formed effectively if the track pitch satisfies TP>3/4×BD, where TP is the track pitch, and BD is the diameter of the light beam, i.e. the size (light spot diameter) of such a part where the light intensity is 1/e2 (e≈2.718) of the maximum value. On the other hand, such patterns that fall within the range of TP<3/4×BD cannot be formed. Therefore, if the diameter of the light spot is about 500 nm, it is impossible to form patterns whose TP is about 375 nm or smaller.
The problem of the method of publication 2 is as follows: a very expensive manufacturing device (e.g. a vacuum chamber for generating ultrahigh vacuum, an electron gun for emitting an electron beam) is required; that is, existing manufacturing devices cannot be used.